How can i become "immune" to g-forces/airsickness?

Wear a G-suit?: Seriously, are you asking about nausea and vomiting due to motion sickness in general, or are you in an occupation that requires repeated exposure to G-forces (e.g. jet fighter pilot) for which you'd like to build a tolerance?

G force: The effects of G forces can be minimized by improving your cardiovascular fitness and learning techniques like the 'hick' maneuver to prevent loss of consciousness in high G environments.